The overall goal of this project is to understand CPT-11 processing and metabolism at the atomic level, and use that information to improve the efficacy and systemic tolerance of this proven anticancer drug. CPT- 11 (Irinotecan, Camptosar(R)) is a potent chemotherapeutic used to treat solid malignancies of the colon, lung, and brain, as well as refractory forms of leukemia and lymphoma. CPT-11 is a prodrug that is hydrolyzed in vivo to its active metabolite, SN-38, by carboxylesterase enzymes (CEs). There are two primary human carboxylesterases: hCE1, predominantly found in the liver, and the intestinal hiCE enzyme. We will determine hCE1 crystal structures and design enzyme variants with improved CPT-11 activation properties. We will also elucidate the first crystal structures of hiCE to unravel the structural basis of its CPT-11 hydrolysis activity. These studies are directed toward developing a novel enzyme-prodrug approach to improve CPT-11 efficacy. The dose-limiting side effect of CPT-11 is acute diarrhea caused by the generation of active SN-38 in the GI. The enzymes responsible for the conversion of CPT-11 to SN-38 are hiCE and the bacterial b- glucuronidases expressed by intestinal microorganisms. To slow this process, we will pursue two separate paths. First, we will use our structural understanding of drug activation by hiCE to advance the development of hiCE-specific inhibitors. Second, in a new direction for this project, we have identified novel inhibitors of b- glucuronidases and tested them against several strains of bacteria known to populate the human GI. We have shown in preliminary studies that such compounds specifically disrupt b-glucuronidase activity with mM to sub- nM potency in Gram-positive and Gram-negative bacteria grown under aerobic and anaerobic conditions. We have also determined the first crystal structure of a bacterial b-glucuronidase. We will now conduct structure- function and chemical biology studies to develop compounds that prevent the reactivation of SN-38 by enteric bacteria, with the goal of controlling the dose-limiting diarrhea associated with CPT-11. We will employ the tools of structural, molecular, cellular and chemical biology to accomplish three specific aims: 1. Understand and improve the processing of CPT-11 by hCE1. 2. Understand and limit the activation of CPT-11 by hiCE. 3. Prevent the intestinal reactivation of CPT-11 by bacterial b-glucuronidases.